Cationic starches and other cationic polysaccharides are widely used and useful additives in paper industry. Fiber, filler and fines material in the paper making have typically negative (anionic) net surface charge. The affinity of polysaccharide to negatively charged material will improve if the polysaccharide contains positive charges in the polymer backbone. Cationic polysaccharides will adsorp stronger onto fiber, filler and fines material of the stock than noncationic polymer. Stronger adsorption can be achieved due to electrostatic attraction forces of negative and positive charges in aqueous solution. Conventional cationic polysaccharides, e.g. cationic starches, which have typically rather low substitution level (DS below 0.06) and are produced as dry powder form, are widely used as dry strength agents in the wet end applications, as surface sizes and in some cases even as coating binders.
Many operations in papermaking will increase the amount of anionic material of the stock. Such are for example closing of the water circulation systems, which will increase the concentration of anionic material. Also hydrogen peroxide bleaching liberates anionic compounds from hemicellulose in to the stock. Plenty of anionic material comes from mechanical pulp (pitch compounds) and from re-pulped coated paper (white pitch). Within such stocks very high cationic starches can be used as ATC-agents (anionic-trash-collectors) and as anti-dusting-agents. In such cases the DS levels are from 0.1 to even up to 1.0. In such cases the performance of cationic starches is more like a performance of cationic polymer rather than the performance of starch. Also the increasing prices of oil based polymers make the natural polymers more attractive.
It is standard practise to manufacture paper by a process that comprises flocculating a cellulosic thin stock by the addition of polymeric retention aid and then draining the flocculated suspension through a moving screen (often referred to as machine wire) to form a wet sheet, which is then dried. Some polymers tend to generate rather coarse flocs and although retention and drainage may be good, unfortunately the formation and the rate of drying may be impaired. The concept of formation is an indication of the arrangement of fibres and fillers within the sheet. A very uniform arrangement is referred to as good formation and is generally associated with better printability, opacity and stability of the paper but also with slow drainage [e.g. Vaughan, Adamsky F. A., Richardson P. F., Zweikomponenten-Hilfsmittel für Entwässerung/Retention/Blattbildung verbessert Produktivität and Runnability der Papiermaschine, Wochenblatt für Papierfabrikation Ser. No. 10/1998, 458-471]. In contrast, an uneven distribution of fibres and fillers is usually regarded as poor formation, but it tends to be associated with rapid drainage. It is often difficult to obtain the optimum balance between retention, drainage and formation by adding a single polymeric retention aid and it is therefore common practise to add two separate materials in sequence, such as a high molecular weight flocculant followed by siliceous material. However, this approach causes an increased complexity of the retention and drainage system, and thus it is often desirable to achieve an optimum balance between retention, drainage and formation with a single polymeric retention aid or even to further improve this balance within dual or multi component retention and drainage systems.
Furthermore producers of paper in contact with food are always looking for retention and drainage aids with less toxic cationic monomer and less residual monomer thereof in the additive. In consideration of continuously increasing scarcity of oil it would also be desirable to use natural polymers in this regard.
There exist many different botanical polysaccharides, which are commercially used in paper making and which thus have commercial importance. Such are for example starch from potato, tapioca, wheat, corn, waxy-corn and oats, natural galacto-glucomannans of wood material, guar gum etc. Practically all of the polysaccharides can be cationised using the same or similar technique. Even though starch is used as a reference here, also other polysaccharides can thus be used as well.
The basic chemistry of starch cationisation as well as cationisation of other polysaccharides is well known in literature. The topic is clarified in many books and articles e.g. O. B. Wurzburg: Modified Starches: Properties and Uses (1986), pages 113-124. Even though literature and patent publications knows several compounds that can be used as a cationising agent for polysaccharide, there exists practically two compounds, which have commercial importance. One is 2,3-epoxypropyltrimethylammonium chloride (EPTAC) and the other is 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC). Both of the chemicals are commercially available in concentrated aqueous solutions. The products have typically purity around 92-97% calculated from the nonaqueous material. There exist some inactive secondary products in the commercial products such as bis-(trimethylammonium chloride)-2-hydroxypropane (BISQUAT) and 2,3-dihydroxypropyltrimethylammonium chloride (DHPTAC). BISQUAT is secondary reaction compound of the forming reaction of the both active compound. DHPTAC is the hydrolysis compound of EPTAC. EPTAC is not totally stable in aqueous solution, but hydrolyses slowly to DHPTAC. There exist differences in both compositions and total quantity of secondary compounds within different commercial cationising agents. Commercial EPTAC products are for example Raisacat 151 and Quab 151. Commercial CHPTAC products are for example Raisacat 188, Quab 188 and Quat 188.
With the above mentioned commercial cationising chemicals, the cationisation reaction is exactly the chemical reaction between starch alcoxide ion and the epoxide ring of EPTAC and the reaction will yield to formation of starch ether. Cationic group of EPTAC will thus be covalently bonded into starch backbone. The reaction is catalysed by base. Base is needed to generate the starch alcoxide-ion. Typical catalyst is sodium hydroxide, but other bases like potassium hydroxide, lithium hydroxide or calcium hydroxide can be used as well. Typical catalyst concentrations are around 0.5-4.0 w-% calculated of the quantity of dry (100%) starch. In the use of CHPTAC as a cationising agent, EPTAC is still the compound, which finally reacts with starch. CHPTAC must be converted first to EPTAC before the cationisation reaction can start. That can be done by adding extra equivalent of base, typically sodium hydroxide, which reacts with CHPTAC and converts it to EPTAC via ring closure reaction of the chlorohydrin group of CHPTAC. Equivalent of chloride salt of the base will be liberate at the same time. The conversion typically takes place “in situ” in the reaction mixture. When EPTAC is formed, the cationisation reaction goes in the same manner as when EPTAC is used as a cationising agent. This means that CHPTAC requires an extra equivalent of base for the conversion to EPTAC, but also a catalytic quantity to generate the required starch alcoxide. Also an extra equivalent of chloride salt of the base will remain in the reaction mixture.
Polysaccharides are not exact compounds but polymeric material which have distribution in molecular weight. In order to define substitution level of cationised products the term degree of substitution (DS) is commonly used. DS is the result of substituted saccharide units divided by total saccharide units. Saccharide units have variable quantities of hydroxyl groups into which cationising agents can react. For example anhydroglucose (AHG) units of starch amylose have 3 hydroxyl groups and thus the theoretical maximum DS is 3.0. Practical maximum is lower because steric hindrance of the substituents. Cationic starch of DS 1.0 has 1 cationic group in every AHG unit in average. Cationic starch of DS 0.1 has thus 1 cationic substituent in every 10th AHG unit in average.
Substitution level of cationic starch can be calculated in many ways. The most typical way is to calculate it form the nitrogen content of pure dry cationic starch. In such case the DS can be calculated with the following equation:DS=N−%×162/(1400−N−%×151.6)
N-content can be determined e.g. by commonly known Kjeldahl-method. The value 162 in the equation is molecular weight of starch AHG. If other polysaccharide than starch is used then the average molecular weight of the saccharide units must be used. The value 151.6 is the molecular weight of EPTAC. Thus for example cationic starch with nitrogen content of 3.5% has DS 0.65.
The term molar ratio (MR) is used to define the molar quantity of cationising agent compared to molar quantity of saccharide unit of the polysaccharide in the cationisation reaction mixture. For example MR 0.1 means that the reaction mixture contains 1 cationising agent molecule for each 10 saccharide units of the polysaccharide. The yield of the cationisation reaction can be calculated thus with the following equation:Yield=(DS/MR)×100%
In the cationisation reaction the EPTAC remnant will be covalently bonded into starch and can't be removed e.g. by washing. On the other hand if EPTAC hasn't reacted with starch but is e.g. hydrolysed, it is not covalently bonded into starch and can be washed away. In order to define the cationic purity of cationic starch product, a bound nitrogen index (BNI) can be used. The BNI value can be calculated from bound nitrogen content of cationic starch and total nitrogen content of cationic starch. The bound nitrogen content is the nitrogen content of pure cationic starch, in which nitrogen is covalently bonded into starch. The total nitrogen content is the nitrogen content of cationic starch product, calculated from dry solids material, which contains also the unbound quaternary ammonium compound i.e. secondary products of the cationising agent including possible unreacted cationising agent. BNI value can be calculated from the following equation:BNI=(N−%Bound×162/(1400−N−%Bound×151.6))/(N−%Total×162/(1400−N−%Total×151.6),i.e.BNI=DS/MRN−Total 
The term MRN−Total is post-calculated “molar ratio”, calculated from the total nitrogen content of dry un-washed product.
If all of the nitrogen is bound, the BNI value is 1, if none of the nitrogen is bound the BNI value is 0, other wise the BNI value is between 0 and 1. If the reaction yield is 75%, the BNI value is 0.75 if the purity of the cationising agent has been 100%. As the purities of the commercial cationising agents are lower than 100%, the BNI value with 75% yield is lower than 0.75. It must be observed that the BNI value differs from the cationisation yield value in a way that the secondary compounds, which exist in the cationising agent before the start of the cationisation reaction, also impact on the BNI value. In addition, the spirit of BNI value is that other nitrogen containing compounds than those originating form cationising agent, which impact on total nitrogen content value, are not counted. Such compounds are e.g. urea which can be used in starch solutions as viscosity control agent.
There are many commonly known processes for cationisation of starch, which are also described in the already mentioned book by O. B. Wurzburg. Such are for example slurry process (wet process), dry cationisation process and gel cationisation process. In the slurry process starch is in slurry form, which have dry solids content up to 44%, into which cationising reagent is dosed and pH is kept alkaline, typically between 10.5-12 at 35-45° C. Starch will remain in granular form during the whole reaction. When cationisation reaction is complete, the reaction mixture is typically neutralised, after which starch slurry is filtered, optionally washed and then dried to the target dry substance content level. Final product is thus starch powder. However, there are some weaknesses with the process. Maximum degree of substitution (DS) is about 0.06. Above that DS level, starch granules start to swell and even partly gelatinise due to adequate cationicity and such starch slurry is difficult to filter and thus powder like products are not possible to produce. Also DS level of 0.06 is low for the market needs at present. In addition cationisation yield of the process, which is around 85% in the maximum, is not sufficient.
The main reason for inadequate reaction yield with slurry process is due to hydrolysis of EPTAC to DHPTAC because of plenty of water present in the reaction mixture. In order to avoid the hydrolysis of EPTAC and achieve better process yield, water content in the reaction mixture has been decreased. That can be done technically with the dry process. There exists several articles and patents about the dry process. For example an article by Hellwig et al. (Production of Cationic Starch Ethers Using an Improved Dry Process, Starch/Stärke 44 (1992) 69-74) describes the improved dry cationising method, i.e. so called Powdercat Process. It is distinguished by short reaction time in the reactor, which is in the case rather a mixer. The reaction mixture is agitated for up to 30 min, after which the reaction mixture is transferred to a storage bin for complete reaction. Reaction yields up to 95% have been reached with potato, tapioca and corn starch. Also DHPTAC concentration with the dry process is much lower compared to concentration with slurry process. The maximum DS level which can be produced with dry process in one reaction step is 0.5. It is said that even higher DS level can be attained but that it requires several consecutive reaction steps. This means that already cationised starch is used as a raw material which is then cationised further. Also production in aqueous medium leads to considerable problems e.g. with the isolation of the cationic starch. This is very complicated in production point of view.
Also patent publication GB-A-2063282 describes the dry cationisation process. Starch is dosed into suitable reactor e.g. Lödige-type reactor, in dry powder form, cationising agent, e.g. EPTAC or CHPTAC is introduced into starch powder after which catalyst is also introduced. Reaction mixture is heated up 20-90° C., optimally 60-80° C. and cationisation reaction takes place. The product will remain in dry powder form during the cationisation reaction. When reaction is completed, the product is neutralised. Process yields of about 100% have been mentioned. The maximum DS level of 0.47 (N-content 2.8%) was achieved.
Patent publication U.S. Pat. No. 4,785,087 describes dry process in which finely divided silica is used in the activator mixture. Good yields have been achieved, but the maximum DS with the process is only 0.3.
An issue which impacts on the processing of cationic starch mixtures of DS above 0.5, is the dissolving or pasting of the starch granules, due to which dry process is not applicable. However there exist processes to produce cationic starches of DS above 0.5. Patent publication WO-A1-9518157 describes the so called gel-cationising process for production of high cationic starches. The method describes cationisation of starch in which solids content of the reaction mixture is over 50%. Reaction mixture is heated to 60° C. and the reaction mixture turns to gel like matrix. Cationic starch of DS 0.75 has been made with 75% yield. However the method requires degrading/splitting/thinning (i.e. lowering of the molecular weight of starch) of starch which is obviously needed to lower the viscosity of the reaction mixture after gelatinisation of starch. Even though the cationisation reaction yield of the process can be considered moderately good, there is a need for better cationisation yield to achieve cationic starches of higher purity.
Patent publication WO-A1-9962957 describes the method for production of high cationic starches with DS of 0.1-1.0 with a method, which contains at least two reaction steps. The method consists of a cold preliminary reaction at 5-40° C., after which there is a rapid elevation of temperature to 70-180° C. and finally a post reaction at temperature lower than 100° C. The reaction yields with the method are mentioned to be between 75-95%. Generally the yield is better with lower DS values and it is mentioned that yield is over 90% with DS values less than 0.4. A cationic starch of DS 0.7 has been produced (experiment 4). MR of the reaction mixture was 0.9 and N-content of the polymer 3.6% (DS 0.7). The reaction yield was thus 75%, which is at the same level than presented in the method described in the patent publication WO-A1-9518157. However there is need for high cationic starches with DS over 0.5 and which have higher purity.
It must be noticed that in the experiment exists a typing error concerning the used raw materials. Experiments lack the concentration of starch and cationizing agent. For example in the experiment 4 MR is said to be 0.9. If starch and EPTAC concentrations would be both 100%, MR would then be 1.0, which is not correct. It is thus obvious that starch is potato starch with moisture content of 18% (equilibrium moisture content). Cationizing agent is clearly commercial EPTAC product with typical concentration of 72%. With these concetrations MR is 0.9 in the experiment 4 and MR's of the other experiments match also very well. It must be observed that the wrong concentrations impact also to the total-water-amounts of examples.
An article of Haack et al. (Macromol. Mater. Eng. 2002, page 495-502) describes the production of highly cationic starches up to DS 1.05. In the process dried starch is suspended in diluted sodium hydroxide and heated to 60° C. EPTAC reagent is added by drops. The reaction time is 6 hours at 60° C. and during the reaction the mixture is diluted with water. Reaction yields with the process are rather poor, varying 23-76%. The lowest reaction yields have been achieved with the highest MR values. The reaction yield with the process is not satisfactory in order to produce high cationic starches with high purity.
It can be summarised that with the known cationisation technology it is not possible to manufacture cationic starches, which have DS above 0.5 and which have adequate purity, in a way which is efficient and has commercial interest.